Jest mock fetch calls. Mar 11, 2025 · Best Practices for Mocking in Unit Tests Using Jest Introduction Mocking is a critical part of unit testing, especially when dealing with external dependencies, APIs, and complex logic. May 15, 2020 · If you are using fetch, you're in the right place, if you are using Axios, head on over here. When writing tests for code that makes API calls, it’s important to mock those calls. fn() to mock the global fetch function, so it doesn’t make actual network requests during the test. This allows you to mock modules, including functions used for making API requests like fetch or axios. mockImplementation((url: string) => getMockPromise({ Response May 15, 2025 · Jest is a popular testing framework for React that supports mocking APIs, modules, and functions. I keep getting errors like: Invalid hook call. This is because the Response class you've imported from node-fetch has been mocked (due to the jest. Apr 3, 2019 · Is there a way to call fetch in a Jest test? I just want to call the live API to make sure it is still working. mock call at the top of the test file) so it no longer behaves the way it should. Jul 26, 2025 · Mocking API calls with Jest is essential for writing reliable, fast tests. fetch = jest . . Hooks can only be called inside of the body of a function component. fn to get the same behavior: By doing this, when the function getPricesLastDays calls fetch, the mocked out version of fetch will be called. Tagged with reactnative, testing, react, mobiledevelopment. We're going to be mocking fetch calls today in our Jest tests, starting with a manual mock, introducing a packing to make it easier and more flexible, and then seeing how we can test React components which rely on remote data. Feb 17, 2023 · If you are calling fetch() with different arguments each time, you can use a mock implementation that responds with different data depending on the argument it receives. mock() method. Aug 14, 2020 · Mocks help get around this problem by reducing a test’s brittleness when calling APIs. mock() or the global fetch function to mock external API calls in your tests. Mock functions are also known as "spies", because they let you spy on the behavior of a function that is called indirectly by some other code, rather than only testing the output. Feb 22, 2021 · We have to mock both promises with jest. Jan 15, 2025 · Mocking API Calls: Use jest. Use its extensive library to control mocked responses and adapters for advanced cases. Jest offers multiple ways to mock API calls, from mocking entire modules with jest. fn() . requireActual helper. mockImplementation(endpoint => P Aug 6, 2019 · In this lesson we'll learn how to mock fetch in your Jest tests. While implementing unit tests using Jest, we often need to mock fetch API because we usually want to test business logics rather than testing this fetch API. You can create a mock function with jest. mock (moduleName, factory, options) to mock node-fetch module and fetch function. Nov 25, 2024 · Conclusion Mocking API calls is a fundamental skill for writing reliable and fast tests, especially when dealing with external dependencies. mock() or global. In order to construct the response object of the fetch function, you need to use the Response class provided by the node-fetch module, so use jest. In this article, I will show you how to mock fetch API, so we can focus on testing only the portion to be tested using Jest. fn(). Mock APIs using jest. Jest runs in a Node-like setup, which—unlike browsers—doesn't have fetch built-in. fetch to simulate responses, test errors, and handle edge cases. Dec 30, 2020 · Why should we mock the network? We'll take a look at why it's important to mock window. Nov 25, 2024 · Mocking API calls with Jest is crucial for writing efficient, fast, and reliable tests. Developers can mimic API calls without actually reaching endpoints by using the robust retrieve request mimicking facilities provided by Jest, a well-known JavaScript testing framework. requireActual (moduleName) to get the original, unmocked node-fetch Module and Response class. To make the above test work, make the following change to the imports in the test file: Sep 4, 2022 · But I'm struggling to write my own test. Instead, we control what fetch returns. Jest Fetch Mock allows you to easily mock your fetch calls and return the response you need to fake the HTTP requests. But Typescript tells me that I'm not matching the definition for the full Aug 7, 2019 · In this lesson we'll learn how to mock fetch in your Jest tests. Mar 25, 2025 · One of the simplest ways to mock API calls in React tests is by using Jest’s built-in jest. Built-In Mocking and Spies: Jest comes with built-in support for mock functions, modules, and timers, making it easy to test components or functions in isolation without worrying about their dependencies. By following the steps outlined in this guide, you can easily mock fetch calls in your TypeScript projects and improve the quality of your test suite. The definitive how to mock and test fetch requests guide for 2020. In React Native I use fetch to perform network requests, however fetch is not an explicitly required module, so it is seemingly impossible to mock in Jest. Feb 28, 2020 · Mocking and testing fetch requests with Jest Some days your code flows, your fingers fly, and you're god amongst nerds. Oct 31, 2019 · We should use jest. If no implementation is given, the mock function will return undefined when invoked. fetch = jest. Oct 6, 2025 · Learn how to mock API calls in Jest with examples that show how to create mock data, test failures, add delays, and build real-world workflows. Other days you're testing fetch() requests. spyOn(global, 'fetch'). Asynchronous Testing: Use waitFor and findBy queries from React Testing Library to handle Mock functions allow you to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls), capturing instances of constructor functions when instantiated with new, and allowing test-time configuration of return values. To get around problems like this, Jest provides the jest. But the test keeps failing. Even trying to call a method which uses Jest Fetch Mock Fetch is the canonical way to do HTTP requests in the browser, and it can be used in other environments such as React Native. By setting up Jest with tools like @testing-library/react and jest-environment-jsdom, you can create a test environment tailored for React apps. So I'm writing this to save you some time. fetch and a couple methods we can use in our test suites. Mar 18, 2025 · How to Mock Fetch Requests in Jest for API Testing When building modern web applications, testing API interactions is crucial for ensuring robust applications. It's easy to setup and you don't need a library like nock to get going and it uses Jest's built-in support for mocking under the Oct 25, 2024 · If you've ever tried testing API calls in a Jest environment, you might've hit a wall with fetch not working. This tutorial will guide you through the essential techniques to control mocked responses using Jest's extensive library and adapters for advanced scenarios. Mar 25, 2025 · Mocking fetch: We use jest. Creating a mock function allows us to replace slow functions or API calls with something else, and gives us access to extra ways to test our code, such as capturing (and being able to assert against) how the function was May 7, 2024 · It takes a snapshot of a component’s rendered output and ensures that it doesn’t change unexpectedly in future tests. I tried to mock fetch in this way: global. Jun 16, 2020 · I would like to test a small React web app where I use the global fetch method. If there are 500 errors or the data is not what I expect than the test should report Aug 21, 2025 · Learn how to mock API calls in Jest to create reliable tests without waiting for backend services or dealing with unstable network responses. First, a little background Why even test requests? Click through for source Integration testing. Jest Fetch Mock Fetch is the canonical way to do HTTP requests in the browser, and it can be used in other environments such as React Native. Mar 30, 2024 · In JavaScript, fetch is a promise based API for HTTP requests and responses. I built a simple weather app just to test/mock the fetch using jest. Mocks fall under the category of “test doubles” as defined by Martin Fowler. Nov 13, 2020 · What would be my absolute easiest way of mocking fetch using Typescript? I would just like to do something simple like below. //test file const mockedResponses = { 'firstUrl': {}, // first response object 'secondUrl': {}, // second response object } global. mock(), manually mocking modules, to using libraries like axios-mock-adapter for more advanced cases. Also known as the only Oct 31, 2024 · Mocking fetch calls in TypeScript using Jest is essential for writing reliable and efficient tests. 8iohtoklwxzf3uhdfsmaplj09ec3lov9occl0xnsuuafwh7sl